Method and apparatus for generating particle beams

ABSTRACT

A source of atomic or molecular particles includes a source of ionized particles (1), an extraction electrode (2) and an einzel lens (3) to focus a beam of particles. A Wien filter (4) selects particles in said beam having a predetermined velocity and a charge exchange cell (7) neutralizes the ionized particles prior to the extraction of non-ionized particles from the beam.

The invention relates to apparatus for generating atomic beams. Withincreasing demand for fast atom applications for surface analysis andother studies, a pulsed fast atom source is urgently needed. Forexample, in instruments employing time-of-flight techniques and usingfast atoms as their incident projectile a pulsed fast atom source isessential. According to the present invention there is provided a sourceof atomic or molecular particles comprising a source of ionizedparticles, means to remove a beam of said particles from said source,focusing means to focus said beam of particles and filter means toselect particles in said beam having a predetermined velocity.

An embodiment of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 is a schematic section of a pulsed atom source

FIG. 2 is a block circuit diagram illustrating the method of pulsing theatom source of FIG. 1

FIG. 3 is a schematic diagram of an experimental arrangement used forthe measurement of the current characteristics of the atom source ofFIG. 1

FIG. 4 is a graphical representation of the proportion of neutrals in anatom beam at different line pressures

FIG. 5 is a plot showing how the secondary electron coefficient varieswith beam energy

FIG. 6 shows the variation of neutral current with differential pumpingline pressure

FIG. 7 is a schematic diagram showing the experimental arrangement fordivergence measurement of the atom beam

FIG. 8 is a current amplifier used in the measurement of atom beamdivergence

FIGS. 9 to 11 are oscilloscope traces

FIG. 12A is a schematic diagram showing the parameters used in thecalculation of current density and FIG. 12B shows parameters used incurrent distribution

FIG. 13 s the result of a typical computation

FIG. 14 is a schematic diagram showing the geometrical relationship usedin the calculation of beam divergence

FIG. 15 is a schematic diagram of the vacuum system of thetime-of-flight facility

FIG. 16 is a schematic diagram of the electronic system of the facility

FIG. 17 is a modified control unit

FIG. 18 is a typical example of the time-of-flight spectrum of a totalbeam

FIG. 19 is a typical example of the time-of-flight spectrum of a neutralbeam, and

FIG. 20 is a fast atom scattering spectrum for argon atoms incident on agold surface.

Referring now to the drawings, the basic idea of pulsing is to generateions only when a voltage pulse is applied. As shown in FIG. 1, ions arecreated by electron impact in an ionization cell 1. They are thenextracted from the ionization cell by means of an extraction electrode 2and focused immediately by an einzel lens 3. A Wien filter 4 then allowsonly one value of ion velocity to pass. Those ions emerging from thefilter are subsequently deflected at an angle of about 5° from theprevious axis by deflecting electrodes 5. This is necessary becauseneutrals created in that section of the gun may have a wide energyspread. This feature thus serves as a neutral dump. A Bruch telefocuslens 6 is then employed to focus the ions through a charge exchange cell7. Such a lens allows one to include a long length charge exchange cellbetween the lens and a target without losing the focused beam. Theregion occupied by the lens is kept under good vacuum conditions, sothat probability of charge exchange is minimized at this stage. Thecharge exchange cell is so designed that either a resonance or anelectron capture charge exchange process can take place inside: thiscorresponds to a high or low neutral current mode. The exit aperture ofthe cell incorporates a set of deflection plates 8 which remove residualions from the neutral beam and also may be used to scan the ion beamwhen the source operates in an ion mode.

The ion source includes a heated filament 9 and a grid 10. Gas isionized by electron impact. This configuration is particularly suitablefor the pulsing method, simple, and easy to be operated.

Optionally, the atom source may include a stigmator S to correct forastigmatism resulting from non-uniform field effects due to the Wienfilter. The stigmator is positioned immediately after the filter elementand consists of two quadruples displaced by 45° from one another. Byapplication of suitable voltages to the quadruples from an externalpower supply, the direction of the correcting field may be adjusted andastigmatism eliminated before the beam enters the second lens system.

Optionally, also, scanning means may be provided for the atom beam. Thiscomprises X and Y deflection plates D, positioned between the secondlens element and the charge exchange cell. By application of a suitablevoltage to the scanning plates, the ion beam may be displaced in araster scan. The beam then passes through the charge exchange cell wherea proportion is neutralized. Ions in the beam are then removed by theplates 14 at the exit aperture, giving a rastered neutral beam.

Part of the control unit for the source is shown schematically in FIG.2. It includes a filament power supply 21, a grid to filament biasvoltage power supply 22, a high power voltage power supply 23, a highvoltage isolation circuit 24 comprising a diode D, a resistor R2 and acapacitor C and a purpose-selected pulse or impulse generator 25. Thefilament 9 is heated by the filament power supply 21 and gives rise tostable thermionic electron emission. Because the energy of suchelectrons is much less than the ionization energy of any element of gas,no ions are produced and thus no atoms. However, if a voltage across thefilament and grid is provided, the electrons will be accelerated and mayobtain sufficient energy to ionize a gas atom if the voltage is higherthan the threshold of the ionization energy. This voltage is pulsedthrough the high voltage isolation circuit 24. This simple circuit isdesigned to pass a pulse train having frequencies in the range of 10 kHzto 1 MHz without significant degradation of shape, while the values ofthe resistor R and capacitor C are so chosen that more than 90% of thevoltage is dropped across the resistor. A grid to filament bias voltageis required here to pull back the energetic electrons when a pulse fallsto its "ground" level. A zener diode D is included in the earthy side ofthe high voltage isolation circuit. This is to protect the pulsegenerator in case of capacitor breakdown.

It is very important to choose a suitable pulse generator. The generalrequirements are mentioned in FIG. 2. In order to produce a sufficientpulse of ions, the amplitude of the voltage pulse must be greater than100 V, into a load of 50Ω. If a high current is not necessary, thisvoltage can be low provided that the voltage across the grid andfilament is higher that the ionization potential of a gas atom. Pulsewidth is an important parameter in some applications such astime-of-flight measurements: the width determines the resolution of thesystem. Pulses with a width as small as 2ns can be obtained from impulsetype generators. However, because capacitance effect could be importantin the pulsing system employed using such a pulse generator, the widthof the final pulse appearing across the grid may be ˜18ns. Frequency ofthe output pulse train governs the collection coefficient of atime-of-flight system. Frequency as high as 1 MHz is good enough formost applications. Parameters such as pulse height, pulse width andfrequency can be specified according to the specific application.

The second important part of the source is the charge exchange cell. Inorder to have effective neutralization, the cell is designed to be ableto maintain pressure of about 10⁻³ mbar two orders of magnitude higherthan that of other parts of the system, with the exception of theionization cell. Another feature of this charge exchange cell is that itcontains a set of hot filaments 11 and a set of electrodes 12 which arelocated opposite one another and parallel to the trajectory of a beam,i.e. the axis of the cell. It is then possible to neutralize ions by anelectron capture mechanism instead of resonance gas charge exchange.Since the neutralization probability by electron capture is low, thesource operated with this mode can be expected to have only a smallcurrent. However, this may be enough for some of the applications suchas fast atom scattering spectrometry where only one atom from each pulseis required. The advantage of operating in this mode is that it makes itmuch easier to pump down the gas flow in the source so that the specimenchamber pressure is easily kept in ultra high vacuum conditions whichare important to many surface analyses and studies. This pulsed sourcemay also be used to produce ion pulses by non operation of the chargeexchange cell.

Another important feature of this source is that it can be easilyswitched to operate in DC conditions, i.e. to output continuous neutralcurrent (NC mode), ion current (IC mode) or both (NIC mode). In the caseof IC mode, beam scanning can be achieved by using the deflection plates14. Therefore, it is possible to use this source in an ion scatteringspectrometry where an electrostatic analyser is employed, in atom or iondepth profiling or in secondary ion mass spectrometry (SIMS) or FastAtom SIMS applications. The nature of beam depends on the operationmode: when the charge exchange cell is filled with gas and thedeflection voltage is off, or instead of filling gas, the filament andthe electrode inside the cell are operated, output is both ion and atom,while if the deflection voltage is on, output is neutral. Without gasinside the cell, output is ion only. In any case, this function is alsovery important because it permits the use of the same source for surfacetreatment during the experiment.

In order to characterize the fast atom source, measurements have beencarried out to determine the variation of neutral currents with specimenchamber pressure, the proportion of neutrals in the beam and thedivergence of a beam, under various operating conditions.

It is necessary to know the relationship between neutral current andchamber pressure because it is important to maintain chamber vacuum ashigh as possible provided that enough neutral current can be obtained.In addition, the measurement of the neutral proportion of the beam canprovide information of purity of a beam as well as of neutral productionefficiency of the source.

The experimental arrangement is shown schematically in FIG. 3. A Faradaycup 31 is mounted axially opposite the exit aperture 32 of the source33. The cup is so designed that any secondary electrons created byincoming particles cannot escape from the cup. It is also prevented frompicking up electrons outside by shielding. The current measured with apicoammeter M31 is the electron current required to neutralize chargedparticles collected in the cup. With this arrangement, it is thereforepossible to measure ion fraction of a beam. A detection plate 36attached to a manipulator 37 is placed in front of the entrance of thecup. With this, the atom flux may be determined by using the deflectionplates of the source to remove the ion content in a beam. A 12-voltbattery B is used to bias the detection plate so that it preventssecondary electrons from coming back to the plate. Before anymeasurement is made, the source is aligned on axis by adjusting thebellows 13 and focused so that any particle detected by the detectionplate goes into the cup. Those not entering the cup will strike theshielding of the cup and thus give rise to a current reading on themonitoring picoammeter M32. Similarly, if the detection plate is notcompletely rotated away from the beam, a current will be recorded in afurther picoammeter M33 Measurement has been made at eight differentenergies, corresponding to source high voltage range of 1 to 5 kV, ofargon.

To obtain a set of measurements, firstly a value of the source voltageis fixed. Then, the leak valves (not shown) are open to allow argon gasto enter the source until pressure in the differential pumping linereaches a desired value. Subsequently, a neutral equivalent currentI_(a) can be obtained by using the detection plate with usage of thedeflection plates of the source removing ions from the beam. Foraccuracy of the measurement the current is allowed to stabilize forseveral minutes. After this, the voltage to the deflection plates isturned off to allow the total beam to strike the detection plate andthus total beam equivalent current I_(t) can be determined. Followingthis, the detection plate is rotated away from the beam by using themanipulator and the ion current in the beam is measured by monitoringthe Faraday cup current I_(i). The above procedure is then repeated fora range of pressures.

The proportion of neutrals in the beam can now be calculated from thefollowing equation: ##EQU1## Several sets of results were processed andplotted and are shown in FIG. 4. As can be seen, within the range ofexperimental pressures the proportion of neutrals is less than 10%. Itis also shown that this proportion varies with pressure and increasesvery slowly before the source pressure reaches certain values, forexample, Pd=10⁻⁵ mbar. In terms of equivalent current, the maximumobtained for atom is -240 nA.

The variation of neutral current with pressure can also be derived fromthese results. First, the secondary electron emission coefficient γ isdetermined in the following form: ##EQU2## because the total currentconsists of three terms: i.e.

    I.sub.t =I.sub.i +I.sub.c ×γ+I.sub.a

where I_(i) is contributed by the electrons to neutralize ions. I_(i)×γ, by secondary electron and I_(a) equivalent current. Secondly,assuming that the secondary electron emission coefficient is the same asfor ions i.e. equal to γ, the actual atom flux I_(a) ' is determined inthe form of I_(a) '=I_(a) /γ. FIG. 5 shows a plot of γ against ionenergy (measured as a function of voltage E_(o)), whilst the variationin neutral current with pressure is shown in FIG. 6.

Measurements with helium have also been carried out and given resultssimilar to those for argon.

Angular spread is an important parameter in atom scattering measurementsince the energy of a scattered particle, in principle depends on thescattering angle, i.e the angle its trajectory makes with the directionof the incident particle. It has been found that conventionalexperimental methods cannot provide satisfactory information. Forexample, atom currents can easily sputter off a phosphor screen and thusdo not give a homogeneous illuminated image, while a gold-coated windowreveals different shapes of a cross beam section depending on the timetaken in an etching process. For this source it is convenient to measurethe divergence under different lens operating conditions without openingthe vacuum chamber and replacing a detecting or recording device.

A simple apparatus has thus been designed for this measurement andprovided some important information of the atom source. The apparatus isillustrated schematically in FIG. 7. A thin metal wire 71 of diameter of0.1 mm is placed ˜24 cm away from the exit aperture of the source. It ismounted in a holder 72 that is controlled by a micro-adjustable specimenstage, and is electrically insulated from it. It is however electricallyconnected to an input of a current amplifier 73, whose circuitry isshown in detail in FIG. 8. The output of the amplifier is connected tothe γ-input of an analogue storage oscilloscope 74. If there are atomsstriking the wire, secondary electrons are generated and the electroncurrents are amplified and recorded in the oscilloscope. Since thedetected current is very small, of the order of nanoamperes, an FETamplifier 82 is used in the input stage of the amplifier. Furthermore,since the gain of the amplifier is quite high, it is important to screenand earth it properly.

In order to allow the wire to cut across an atom beam, the wire is movedhorizontally by adjusting the specimen stage outside the vacuum. Thismovement is converted to voltage through a potentiometer 75 powered by apower supply 76 and the signal is input to the X-input of theoscilloscope. The movement recorded on the screen of the oscilloscopecan be calibrated precisely by referring to the actual movement showingin the micrometer of the specimen stage.

To measure the divergence, a detected current distribution is firstrecorded. After setting up the source operating in normal conditions,the wire is scanned across the beam 77 by moving the specimen stage 78manually. The distribution is often very broad and may be badlydistorted under these lens conditions. Sometimes distributions withdouble peaks can occur. To obtain the best focusing conditions, it isnecessary to follow the operating guide rules provided by the sourcemanufacturer and adjust the lens voltages every time. FIG. 9 is atypical detected current distribution and is in the form of a Gaussiandistribution. It is found that only one set of lens voltages can giverise to the best focused beam of all different energies of the atom.However, in general the higher the energy of the atom, the less the beamis diverged. This is shown in FIG. 11. Another important finding ispresented in FIG. 10, which shows two distributions corresponding tototal beam and neutral beam respectively. It can be seen that there is adisplacement between two peaks.

With the distributions like that shown in FIG. 9, the true beamdivergence may be calculated by means of a simple mathematical procedurewith the value of the distribution's full width at half maximum.However, in order to calculate the divergence more accurately, a currentdensity distribution is required. In fact, referring to FIG. 12B, acurrent density can be determined according to the following form:##EQU3## where I is the current detected and d the diameter of the wireas shown in FIG. 12A. Since the recorded current distribution is in theGaussian form, I can be determined as below:

    I=H.sub.p ×exp (-F.sup.2 /2.sub.x.sup.2)

where H_(p) is the peak high, F full width at half maximum (FWHM); theycan be measured from the recorded current distribution. FIG. 13 is anexample of this computation result; the inner curve is a simulatedcurrent distribution while the outer the current density distribution.

Referring to the geometric relationship illustrated in FIG. 14, theangle θ representing the beam divergence is determined by the followingrelationship: ##EQU4##

According to the design of the ion optic system of the source, a beamcross disc should locate at ˜1 inch away from the exit aperture so thatL is equal to a term of (24 cm-1 in). Also, in this calculation theconventional idea of using FWHM in such beam divergence estimation isapplied.

The neutral production efficiency of the source is rather low and theneutral current is small, for example, about 10nA at chamber pressure of˜10⁻⁶ torr. However, with our time-of-flight system, it is possible tooperate with the source working in the very low current mode because ofthe high transmission coefficient of such a system. One of the featuresof this source is that it can provide a pure neutral beam. Thiseliminates the possibility of confusion of atom scattering with ionscattering. The most impressive features of this source is its verysmall beam diameter and its divergence which is around 1°. This smallbeam diameter which may be around 350 μm facilitating the sampling ofinteresting areas of a target. Both features ensure a very goodresolution when used in a Fast Atom Scattering Spectrometer (FASS).

Experiments have also been carried out to measure the energydistribution of fast atoms and ions. In order to measure this energy forneutral particles, a time-of-flight technique has been employed in whichthe time taken for a particle to travel freely over a known distance ismeasured accurately. The apparatus for this is shown in FIG. 15 andcomprises a pumping system, analysis chamber and flight tube. Thepumping system, which comprises a rotary pump 150, valve 151, traps 152,153, pirani gauge 154 and diffusion pump 155 with rough line 156,maintains a pressure of less than 10⁻⁹ torr. The analysis chamberincludes an atom source 157, a flight tube 158 provided with an iongauge 159 and detector mounting port 160. In order to obtain good vacuumconditions within the analysis chamber, the source is pumped by aturbo-molecular differential pumping stage comprising a turbo pump 161with an isolation valve 162 and ion gauge 163. The basic electronicsystem designed to accomplish the time-of-flight measurements is shownin FIG. 16 and includes nanosecond pulsing, detection and dataacquisition circuitry.

In order to produce a neutral pulse for the time-of-flight system, it isnecessary to modify the source control unit that only operates for thesource giving continuous neutral current. The circuit of the modifiedcontrol unit is shown in detail in FIG. 17. The major part of it is apower supply to the filament of the source, with a filament overvoltageprotection circuit. The integrated circuit of the IC1 provides afunction of stabilizing the filament current. The feedback of this IC isnow provided by V₁ instead of using electron emission current. Thisfeedback is necessary because otherwise voltage to the filament will beincreased until it is tripped over. With this part of the circuit, thefilament may be heated and gives rise to a stable thermionic electronemission. Because the energy of such electrons is much less than theionization energy of any element of gas, no ion is produced and thus noatoms. However if a voltage across the filament and grid is provided,the electrons will be accelerated and thus obtain enough energy toionize the gas atom, if the voltage is higher than the threshold of theionization energy. This voltage is provided with a pulse transmittedthrough the high voltage isolation circuit enclosed with the dashedlines. This simple RC circuit is required to allow a pulse train havinga frequency in the range of 10 kHz-1 MHz without degrading its shapewhilst the values of the resistor R and capacitor C are so chosen thatmore than 90% of the voltage is dropped across the resistor.

In the measurement of the energy distributions of both the neutral andtotal beams, stop apertures have been placed inside the flight tube toprevent particles scattered inside the tube from reaching the detector.FIG. 18 is a typical time-of-flight energy distribution of the totalbeam. The main spectral peak corresponds to Ar and the smaller peak toAr⁺⁺. The energy spread is ˜1% at the incident particle energy. FIG. 19is the corresponding spectrum for the neutral beam.

Experiments reveal that without the Wien filter residual gas peaks alsooccur, indicating am impure beam.

Improvements may be made in the method of production of monoenergeticfast atoms by introducing both a neutral dump and a Wien velocity filterinto the source.

The FASS technique may also be used to obtain information on thecharacteristics of surfaces. An example of scattering of argon atomsfrom a contaminated gold surface using the FASS is shown in FIG. 20.

As with low energy ion scattering spectrometry, our fast atom scatteringspectrometer will provide surface chemical composition information byanalysis of the spectrum of the scattered atom. But this study may befocused on how to obtain a high resolution spectra and thus involves theelimination of spurious charge effects.

Due to basic scattering mechanisms shadowing effects may be observed inthe spectra. This can be used to study the orientations of the surfaceatom, giving unique information on atomic arrangement in the surface. Bychanging the incident angle of the primary beam amplitudes of spectralpeaks may vary or even some peaks may disappear. Analysis of theseresults can thus provide information on the surface structure.

In experiments with low energy ion scattering spectrometry, it has beenfound that the relationship between scattered ion yield and incident ionenergy varies with the combination of the surface of a target and anincident ion. Bonding information may be obtained by a study ofcharacteristic curves of scattering ion yield.

By operating the time to amplitude converter in coincidence mode, it ispossible to record sputtered species in the multi-channel analyser. Fromthe area of the recorded distribution and time taken, sputter rate maybe calculated. Mass analysis may also be available by incorporating amass filter into the flight tube.

By applying the time-of-flight system to a variety of materials such asmetals, semiconductors and insulators, and using either ion or atoms asbombarding particles, differences of chemical damages caused by thesetwo projectiles may be detected. This is of major interest too, forexample, the semiconductor industry where ion surface modifications arebecoming more and more important.

We claim:
 1. A source of atomic or molecular particles comprising:asource of ionized particles; means for removing a beam of said ionizedparticles from said source; focusing means for focusing said beam ofparticles to form a focused beam of particles; filter means forselecting particles in said focused beam having a predeterminedvelocity; and charge exchange means for permitting neutralization ofcharge on said ionized particles, wherein said charge exchange meansmaintains a pressure at least two orders of magnitude higher than thatof adjacent parts of the system.
 2. A source of atomic or molecularparticles as claimed in claim 1, wherein said source produces a pulsedbeam of ionized particles.
 3. A source of atomic or molecular particlesas claimed in claim 2, wherein said filter means includes a neutral dumpcomprising a Wien filter which allows only one value of ion velocity topass at a given time.
 4. A source of atomic or molecular particles asclaimed in claim 3 further comprising means for deflecting the ionsemerging from the Wien filter.
 5. A source of atomic or molecularparticles as claimed in either claim 1 or 2, wherein said sourceincludes an ionization cell which creates ions in said beam of ionizedparticles by electron impact.
 6. A source of atomic or molecularparticles as claimed in claim 5 further comprising an extractionelectrode for extracting said ions from the ionization cell.
 7. A sourceof atomic or molecular particles as claimed in claim 6 wherein saidfocusing means comprises an einzel lens for focusing said ions afterextraction from the ionization cell.
 8. A source of atomic or molecularparticles as claimed in claim 7, wherein said filter means includes aneutral dump comprising a Wien filter which allows only one value of ionvelocity to pass at a given time.
 9. A source of atomic or molecularparticles as claimed in claim 8 further comprising means for deflectingthe ions emerging from the Wien filter.
 10. A source of atomic ormolecular particles as claimed in claim 9, wherein said deflecting meansdeflects the ions emerging from the Wien filter at an angle of about 5°from the previous.
 11. A source of atomic or molecular particles asclaimed in claim 6, wherein said filter means includes a neutral dumpcomprising a Wien filter which allows only one value of ion velocity topass at a given time.
 12. A source of atomic or molecular particles asclaimed in claim 13 further comprising means for deflecting the ionsemerging from the Wien filter.
 13. A source of atomic or molecularparticles as claimed in claim 5, wherein said filter means includes aneutral dump comprising a Wien filter which allows only one value of iondensity to pass at a given time.
 14. A source of atomic or molecularparticles as claimed in claim 11 further comprising means for deflectingthe ions emerging from the Wien filter.
 15. A source of atomic ormolecular particles as claimed in claim 1 further comprising a Bruchtelefocus lens to focus ions in said focused beam which have passed saidfilter means, through said charge exchange means.
 16. A source of atomicor molecular particles as claimed in claim 15, wherein a region occupiedby the Bruch telefocus lens is held under high vacuum conditions inorder to minimize the probability of charge exchange therein.
 17. Asource of atomic or molecular particles as claimed in claim 1, whereinthe charge exchange means performs one of a resonance or an electroncapture charge exchange process therein.
 18. A source of atomic ormolecular particles as claimed in claim 1, wherein said pressuremaintained by said charge exchange means is about 10⁻³ mbar.
 19. Asource of atomic or molecular particles as claimed in claim 1, whereinthe charge exchange means includes a set of heatable filaments and a setof electrodes which are located opposite one another and substantiallyparallel to a trajectory of a beam to neutralize ions by an electroncapture mechanism.
 20. A source of atomic or molecular particles asclaimed in claim 1, further comprising an exit aperture incorporating aset of deflection plates to remove residual ions from the beam which hasbeen neutralized by said charge exchange means.
 21. A source of atomicor molecular particles as claimed in claim 1, further comprising an exitaperture incorporating a set of deflection plates to scan an ion beam.22. A source of atomic or molecular particles as claimed in claim 1,wherein a source of ionized particles includes a heated filament and agrid.
 23. A source of atomic or molecular particles as claimed in claim1, further comprising stigmator means adjacent said focusing means, tocorrect for astigmatism resulting from non-uniform field effects.
 24. Asource of atomic or molecular particles as claimed in claim 1, whereinthe stigmator means comprises a pair of quadruples displaced by 45° fromone another.
 25. A source of atomic or molecular particles as claimed inclaim 1, further comprising scanning means to generate a raster formatbeam.